The present invention relates to a semiconductor device comprising an integrated circuit with a vertical bipolar transistor having a collector region and an emitter region of a first conductivity type and an intermediate base region of a second conductivity type opposite to the first type, wherein the emitter region has a thickness and doping level such that the diffusion length of the minority charge carriers injected into the emitter during operation is larger than or equal to the thickness of the emitter region and wherein the emitter is covered by an insulating layer which is provided with a contact window through which an electrical contact is provided on an emitter contact region. The emitter contact region might for instance be the part of the emitter region directly adjoining said contact or it can for instance be constituted by an intermediate contact zone of the same conductivity type but of a higher dopant concentration than the emitter region.
In order to obtain a maximum amplification, it is considered hitherto that either only the doping profile of the base or only the doping profile of the emitter can be influenced while maintaining the electrical emitter contact zone on the largest possible emitter surface. This permits of optimizing inter alia the value of the current amplification, which is proportional to the injection efficiency of the emitter-base junction defined by the ratio .lambda..sub.O between the current of minority charge carriers injected into the base and that injected into the emitter.
The present invention is based on the recognition that in the case in which the thickness and the doping level of the emitter region are such that the diffusion length of the minority charge carriers injected vertically into the emitter is larger than the said thickness, the injection phenomena would have to be considered in a quite different manner.
In French Patent FR-2,592,525, the injection phenomenon concerning the lateral transistors has already been envisaged. The basic idea then was to minimize the vertical injection into the base of the transistor in favour of the lateral injection while utilizing the fact that the vertical injection into the emitter is considerably smaller under the surface isolation than under a contact zone. An analysis utilizing a simplified model of the phenomenon then has permitted of determining the optimum conditions of demensions of the emitter contact surface leading to an optimum amplification.
On the contrary, for a vertical transistor, a high amplification can only be obtained by promoting to the highest possible extent the vertical injection into the base, and the preceeding remarks logically lead to the conventional theory according to which the emitter contact zone must occupy the largest possible emitter surface because the injection under a (metal) contact zone is much higher than under the isolation.